1. Field of the Invention
The present invention relates to a magnetic resonance imaging apparatus and a magnetic resonance imaging method which excite nuclear spin of an object magnetically with a RF signal having the Larmor frequency and reconstruct an image based on a magnetic resonance signal generated due to the excitation, and more particularly, to a magnetic resonance imaging apparatus and a magnetic resonance imaging method, which reconstruct an image with suppression or excitation of a magnetic resonance signal from a specific part, such as fat-saturation.
2. Description of the Related Art
Magnetic Resonance Imaging (MRI) is an imaging method which excite nuclear spin of an object set in a static magnetic field with a radio frequency (RF) signal having the Larmor frequency magnetically and reconstruct an image based on a magnetic resonance (MR) signal generated due to the excitation.
In the field of magnetic resonance imaging, MRA (magnetic resonance angiography) is known as a method for obtaining an image of the blood flow at a desired portion, such as the head, lung field, or stomach. The MRA includes enhanced MRA and un-enhanced MRA. The enhanced MRA is an imaging method with a contrast medium injected to an object. The un-enhanced MRA is an imaging method without a contrast medium. In any MRA, importantly, in order to obtain the image of the blood flow, an MR signal from a fat is suppressed and an MR signal from water, serving as a component of the blood flow, is excited, thereby sufficiently obtaining the contrast between a blood flow region and a parenchymal region other than the blood flow region.
Conventionally, a fat-water separation method is used. According to the fat-water separation method, an MR signal (fat signal) from the fat is suppressed by using the difference (chemical shift) in resonant frequencies between protons of fat and water. The fat-water separation method includes a pre-pulse method and a water excitation method. The pre-pulse method has been put into practical use. According to the pre-pulse method, a fat saturation pulse for suppressing a fat signal is applied to the object, as a pre-pulse prior to imaging a blood flow. This pre-pulse selectively excites only the fat depending on frequencies so that protons of the fat are saturated. Subsequently, an imaging of blood flow starts. According to the water excitation method, a water excitation pulse is applied, as an excitation pulse. The improvement in this water excitation pulse enables the excitation of only the MR signal (water signal) from the water without generating the fat signal.
Further, according to the fat-water separation method using the pre-pulse method, in order to prevent a trouble of the reduction in water signal due to the small difference in resonant frequencies between protons of a fat and water, an improvement in a frequency band of a fat saturation pulse to suppress the reduction in a water signal is suggested (see, for example, JP-A-2002-306447).
When a frequency shifted from the resonant frequency of protons of water by 500 Hz is selectively excited by an RF pulse, an MR signal level from protons of a high polymer, serving as a fat component, and an MR signal level from protons of water are reduced respectively. Advantageously, an image with the contrast depending on the rate of high polymers is obtained. In this case, an MR signal level at the fat region is excessively reduced, as compared with an MR signal level at the blood flow region.
In order to obtain the above-mentioned MT (magnetization transfer) advantages, a technology for applying an RF pulse, so-called MTC (magnetization transfer contrast) pulse, as a pre-pulse to the object prior to imaging, is proposed. This technology is applied to an MRA with the extraction of a little blood vessel (see, for example, JP-A-H06-319715).
Further, based on three dimensional (3D) image data obtained with various improving technologies of contrast for applying a fat saturation pulse, a water excitation pulse, and an MTC pulse, imaging processing such as MIP (maximum intensity projection) processing generates three dimensional image data for diagnosis, such as an MIP image.
A fat saturation or a water excitation under the conventional MRA uses the difference (chemical shift) in resonant frequencies between protons of a fat and water. Therefore, when a region as an imaging target includes an uneven magnetic field, there is a problem that a fat signal cannot be properly suppressed or a water signal cannot be properly excited. In particular, in the case of MRA of the head of the object, the region near a curve portion of a blood vessel passing through the bone portion ranging from a carotid pyramidal portion to a syphone partly includes uneven magnetic field. Therefore, the fat signal is not preferably suppressed and the water signal is not preferably excited. The MR signal from the water component is lost and an MRA image with the lost blood flow region may be generated.
Currently, the improvement in sequence waveform, serving as an imaging condition, cannot solve the problem regarding as the loss of the blood flow region on the MRA image obtained in a region having an uneven magnetic field.
Then, when the uneven magnetic field is not ignored and a fat signal cannot be preferably suppressed or a water signal cannot be preferably excited, the image is obtained without the fat saturation or the water excitation. Then, the image data as an imaging result is subjected to imaging processing, such as region processing, thereby generating image data suitable for diagnosis.
For example, in the case of the MRA of the head for generating a blood flow image of the head, a fat region near a scalp does not enable the extraction of blood vessel. Therefore, the fat region needs to be removed from the image data. Then, for the image data obtained by the imaging without the fat saturation of head, an inner region of the scalp is set as a region of interest (ROI), thereby removing the peripheral fat region including the scalp from the image data. Then, only the image data at the inner region of the scalp is subjected to specific MIP processing, such as Partial-MIP processing, thereby generating the image data suitable to the diagnosis.
As a consequence, a troublesome operation including the setting of an ROI and the removal of image data at the fat region is needed. In particular, in Whole Brain MRA, the operation including the setting of an ROI and the removal of image data at the fat region is more complicated and further impossible.
That is, the conventional technologies of the fat saturation and water excitation cannot be applied to a portion, such as a curve portion with an uneven magnetic field particularly, and need specific image processing, such as Partial MIP for generating an image data for diagnosis. In other words, in the case of obtaining an MRA image at a portion such as a curve portion, the conventional technologies of the fat saturation and water excitation have a problem that it is not possible to obtain the original merit of the conventional technologies of the fat saturation and water excitation, that is, a merit for obtaining an MRA image with a small loss of the blood vessel region at a wide area without specific imaging processing.